A typical gas turbine engine has an annular axially extending flow path for conducting working fluid sequentially through a compressor section, a combustion section, and a turbine section. The compressor section includes a plurality of rotating blades which add energy to the working fluid. The working fluid exits the compressor section and enters the combustion section. Fuel is mixed with the compressed working fluid and the mixture is ignited to add more energy to the working fluid. The resulting products of combustion are then expanded through the turbine section. The turbine section includes another plurality of rotating blades which extract energy from the expanding fluid. A portion of this extracted energy is transferred back to the compressor section via a rotor shaft interconnecting the compressor section and turbine section. The remainder of the energy extracted may be used for other functions.
Each of the rotor blades includes an airfoil portion, a root portion, and a platform. The airfoil portion extends through the flow path and interacts with working fluid to transfer energy between the rotor blade and working fluid. The root portion engages the attachment means of the disk. The platform is typically integral to the rotor blade and extends laterally from the rotor blade to a platform of an adjacent rotor blade. The platform is disposed radially between the airfoil portion and the root portion. The platform includes a radially outward facing flow surface. The plurality of platforms extends circumferentially about the longitudinal axis of the gas turbine engine to define a radially inner flow surface for working fluid. This inner flow surface confines working fluid to the airfoil portion of the rotor blade.
As a result of the rotation of the rotor blade and its lateral extension, the region of the attachment of the platform to the root portion and airfoil portion of the rotor blade is subject to significant stress. To accommodate this stress, a fillet is located in the region of the attachment to prevent a stress concentration from occurring. Additionally, the fillet provides an aerodynamically smooth transition between the platform and the airfoil portion of the rotor blade.
A significant amount of cracking has been noticed to occur in the fillet, especially in the region of the leading edge of the airfoil portion. The cracks may result in the rotor blade being replaced at a higher frequency than desired. It is believed that high temperatures encountered by the fillet along the leading edge heat up the fillet. The heating of the fillet results in significant thermal stress and reduces the allowable stress of the fillet below an acceptable level.
Placing cooling holes in or near the fillet to provide film cooling over the fillet is a possible solution. The flow dynamics in the region of the leading edge fillet, however, make this difficult to do successfully. In addition, placing cooling holes through the fillet surface may increase the local stress because of the stress concentration associated with each hole.
The above art notwithstanding, scientists and engineers under the direction of Applicants' Assignee are working to develop effective cooling means for rotor blade platforms.